When constructing pipe systems using pipes, shaped members and valves made from thermoplastic synthetic resin, it is clearly necessary to interconnect the individual parts. Welded joints are commonly used for such connections. A special type of welded joint is obtained by forcing a sleeve-like welding muff with a resistance heating wire inserted in the muff body onto the pipe sockets to be interconnected, after which the resistance heating wire is heated electrically so that both the material of the pipe socket and the muff are partly softened and coalesce until an intimate connection is formed. After cooling, a gas and liquid-tight weld is obtained.
Welding using such welding muff is performed with welding devices which either operate with low voltages, that is up to about 60 volts, or with mains or line voltage, in which case the muffs are effectively operated at voltages below 185 volts. The reason for this is that the welding and metering devices are able to regulate and stablize mains voltage fluctuations. However, the expenditure needed for the electrical components, primarily devices for regulating mains voltage fluctuations occur with such welding and metering devices, is considerable and they are therefore not well suited for use on building sites. When they are used for this purpose, faults often occur in such devices which can lead to defective welds. In addition, the welding times must be very precisely adhered to with the known welding and metering devices. It is necessary in the known welding muffs in which the resistance range of the wires inserted in the muff body varies from approximately 0.1 to approximately 30 ohms, depending upon the size of the diameter of the joint, have a temperature which does not differ too greatly from the basic normal temperature. Otherwise, defective welds must be expected. Thus, if the muff temperature is too low, the components will be inadequately welded. It is true that the possibility of defective welds can be counteracted to a certain extent by giving the muff body a shrinkage reserve, i.e., by expanding it. When using this technique, if an inadequate weld occurs, there will at least be a shrink joint of the components which, although possibly adequate in the case of unpressured pipe systems when no additional stresses exist, such welds are not adequate with pipe systems which are subject to stress and pressure.